Silicon Microspheres as UV, Visible and Infrared Filters for Cosmetics

As is generally known, the solar radiation spectrum extends from wavelengths of 200 nm to 3,000 nm. Within this spectrum, the different types of radiation can be classified by wavelength and energy content. Ultraviolet (UV) radiation, from 200 nm to 380 nm, produces burning and erythema, in the case of UVB, or skin aging and photocarcinogenesis possibly leading to skin cancer, in the case of UVA.¹ Visible light falls in the range of 380 nm to 700 nm; and infrared (IR) radiation, from 700 nm to 3,000 nm, is responsible for heat and also appears to be involved in skin aging^{2, 3} and cancer.⁴

Solar radiation that reaches the earth’s surface is composed of approximately 7% UV. The remaining 93% is roughly divided between visible and IR radiation. While 7% is seemingly small in comparison, this level of UV radiation is sufficient to cause skin damage. Moreover, the ozone layer depletion during the past few decades has enhanced the levels of UV radiation that reach the Earth’ s surface.⁵ Therefore, efforts have been devoted to the development of organic and inorganic UV filters, and the sunscreen industry has benefited from the introduction of new active ingredients to enhance UV protection.⁶

Organic chemical molecules including salicylates, cinnamates, camphor, triazone derivatives (UVB) or benzophenones, avobenzone, bemotrizimol (UVA), etc. absorb UV radiation, whereas inorganic particulates like titanium dioxide (TiO₂) and zinc oxide (ZnO) reflect and scatter UV rays. Micronized particles of these latter compounds have been used to improve protection against UVA since they scatter light efficiently in the 320–400 nm range, provided they are present in sufficient quantities. Cosmetic manufacturers currently use these materials in conjunction with organic UV-absorbing chemicals to boost sun protection in the UVA region or to broaden the spectral coverage. However, while these particles effectively scatter UV, their use in sun care formulas poses a challenge since they appear white on skin. This is due to the attenuation of visible light, which is particle-size dependent, and is generally aesthetically unacceptable.

To avoid this whitening effect, nano-sized particles were developed that provide acceptable UV protection while appearing much more transparent on skin, a characteristic not achievable with larger particles. Moreover, after controversy over the possible skin penetration of nanoparticles and their localization in lymph nodes,^7–9 recent research finds that nanoparticles such as TiO₂ or ZnO that are used in cosmetics do not penetrate the skin beyond the stratum corneum.¹⁰ In addition, several studies show that due to strong forces between nanoparticles—e.g., van der Waals and electrostatic forces, and hydrogen bonding and water molecules bridging between particles—aggregates tend to form during sunscreen preparation, thus resulting in clusters of particles too large to penetrate the stratum corneum.^{11, 12}

However, reducing the crystal size of particles, particularly TiO₂, may lead to an enhancement of the photocatalytic activity^13–16 and could induce free radical formation in the presence of light. Therefore, many manipulations such as coatings, pre-treatments, pre-dispersions, etc., are necessary to minimize this effect^{17, 18} as well as improve the dispersion quality and compatibility of TiO₂ and ZnO particles in cosmetic formulations.¹⁹

Sun care research in the cosmetics industry has thus far focused mainly on the damaging effects of UV radiation on human skin and the development of UV filters for cosmetic formulas. However, radiation in the visible and IR ranges also should be taken into account since it is absorbed by the human skin.^{20, 21} Also, as described above, the total direct radiation to which individuals are exposed is approximately 39% in the visible range and 54% in the IR range—a fairly significant amount.

It is important to note that little is known about the biological effects of IR radiation on skin²² and while some studies indicate that it may be involved in the premature aging of skin^{21, 23, 24} and, in synergy with UV radiation, the development of skin cancer,^{4, 25} others show that IR radiation may have a protective effect against UV radiation damage.²⁶ Regardless, the presence of any IR radiation corresponds to heat radiation, which is absorbed by the epidermal layer and causes an increase in skin temperature²² as well as lesions and carcinogenesis.²⁷ Therefore, interest has grown in studying and developing sunscreens that protect against the thermal effects of IR radiation.

Silicon Microparticles: Synthesis and Optical Properties

Recently, the authors developed silicon polydispersed microparticles using chemical vapor deposition (CVD) techniques (see CVD Techniques). Such microparticles are synthesized by the decomposition at high temperatures, i.e., from 400°C to 800°C, of disilane (Si₂H₆) gas as a precursor.^28–30 Under controlled chemical reaction conditions, silicon seeds form and particles grow at a micrometric size. The polydispersed microparticles obtained were assessed for their shape, smoothness, refractive index and size for potential benefits in sun care formulas.

Shape: Particle shapes can confer optical properties to sun care formulas. Therefore, the silicon microparticles were examined for consistent shape by SEM.^{28, 29} Their spherical shapes were found to range from 0.3 to 5.0 microns^a. Figure 1 shows a scanning electronic microscopy (SEM) image of a 2-micrometer silicon microparticle. Indeed, this spherical morphology enhances light scattering (data not shown), indicating the material could provide such benefits in sun care.

Smoothness: The microparticles were then examined for smoothness by both SEM and atomic force microscopy (AFM).^{28, 29} The surface roughness of the silicon microparticles was found to range from 0.2 nm to 0.6 nm, making them optically flat.²⁹ Such smoothness properties would additionally confer a soft feeling to skin upon the application sun care products.

Refractive index: The refractive index of the microparticles can be assimilated to the silicon polycrystalline data tables of Palik (n = 3.5).³¹ This high refractive index value means a high refractive index contrast between the silicon microspheres and the surrounding medium (n ≈ 1.3 in formulations). Therefore, in sun care formulas, silicon microparticles would efficiently scatter light, providing radiation protection.

Size: The size of the silicon micro-particles was examined by SEM and found to range from 0.5 microns to 5.0 microns. Size distribution of the colloids was determined by the mean of an algorithm^b to find the diameter of spheres on several high magnification SEM images. A particle size distribution on the order of microns would avoid potential skin penetration. Figure 2 shows the size distribution of the silicon microparticles in a sunscreen sample developed for the present study, described below, which centered around 1.8 microns.

Due to the morphological characteristics and a high refractive index of these microparticles, each single particle behaves as an optical microcavity in which visible and IR radiation are trapped and then scattered.^28–30 Depending on reaction conditions, silicon microparticles appear as isolated particles, forming clusters of several units. Figure 3a shows an SEM image of a 60-micrometer thick ensemble consisting of a disordered network of polycrystalline silicon microspheres of different sizes deposited on a substrate during synthesis. The authors refer to such aggregations as photonic sponges due to their scattering properties. Figure 3b shows the transmittance spectrum of the silicon sponge ensemble. For comparison, the transmittance spectrum of a silicon wafer slice of the same thickness also is shown.

To further assess the ability of the microparticles to block IR and UV radiation in formulations, sample sunscreens were developed and their transmission measurements taken.

Materials and Methods

Emulsion formulation ingredients for test sunscreens were obtained^c and for the sake of comparison, samples containing 30 nm-TiO₂ nanoparticles^d were prepared. UV transmission measurements were performed using PMMA plates and substrates^e of 6-micrometers roughness on a homemade optical setup equipped with: an integrated sphere to collect the overall light radiated by the samples in all directions, a monochromator^f, a xenon lamp^g and a silicon detector^h.

IR optical transmissions of test samples were carried out on a Fourier transform IR spectrophotometer (FT-IR)^j. Standard amounts of 2 mg of preparation per cm² of substrate were used in all measurements. The test sunscreen with silicon microparticle dispersions³³ was prepared according to Formula 1a, while the TiO₂-containing preparation was produced according to Formula 1b. Thermal protection measurements were performed by irradiating samples with a xenon lampg emitting a continuous spectrum from the UV to IR range, and measuring the temperature with an IR temperature sensor.

Results and Discussion

As Figure 4 shows, the optical transmission of the sample formula containing the silicon microspheres was similar to that of TiO₂-based solar filters in the UV region. Thus, the silicon microspheres efficiently blocked visible and UV radiation, suggesting their application as radiation filters that are efficient across a broad wavelength range; i.e. from UV to visible and IR regions. Moreover, silicon is a biocompatible and chemically inert material;³² and, as noted, the micrometric size of the particles described here would not penetrate the skin.

In addition to UV radiation, the silicon microspheres were found to block IR radiation when deposited as a photonic sponge with a threshold wavelength value dependent upon the mean value of the particle diameters.^{6, 7, 28–30} Figure 5 shows the transmittance of the silicon microsphere-based sunscreen in the IR region, compared with the control formula containing TiO₂ particles. The silicon microsphere-based filters were substantially more efficient at blocking IR radiation—up to several microns of wavelength.³³ The mechanism behind this pronounced blocking effect is the strong scattering efficiency of the spherical silicon particles due to their high refractive index value (n = 3.5).

Furthermore, in regard to thermal protection, experiments were conducted to test the behavior of the silicon photonic sponges when deposited on a substrate that was then heated to approximately 100°C. The temperatures of two surfaces—i.e., one coated with 2 mg/cm² of the test sunscreen with 1% silicon particles, and the other coated with the preparation without silicon microparticles—was measured. Results indicated a decrease in temperature of approximately 10–12% in the regions covered by the silicon photonic sponge (data not shown). This indicates the silicon microparticles were able to scatter heat radiation.

Safety Assessment

Silicon is a biocompatible and chemically inert material;³² in addition, the micrometric-sized particles described here would be unable to penetrate the skin. To substantiate the safety of the silicon microspheres in a test formula (see Formula 1a), clinical assessments of their cutaneous compatibility³⁴ as well as tolerance studies with patch tests were carried out with 20 volunteers, all having Fitzpatrick skin types from I–V, under dermatological control. Two grams of the silicon microsphere-based emulsion were applied on the backs of the volunteers under an occlusive patch and none showed cutaneous response after 24 hr (data not shown).

Conclusions

By optical transmission measurments, silicon microspheres proved to be good candidates for developing effective broad-spectrum radiation filters—from IR to visible and UV. These high refractive materials represent a new generation of solar filters in that they are chemically inert, able to absorb UV and visible rays, and strongly scatter IR radiation. Moreover, the spherical shape and smoothness of the silicon microspheres can impart desirable properties in cosmetic preparations, providing a soft feel when applied onto the skin. Finally, their high IR blocking power enables protection from thermal effects, suggesting new benefits for sun care products.

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